The control of friction and wear of metal mechanical components that are in sliding or rolling-sliding contact is of great importance in the design and operation of many machines and mechanical systems. For example, many steel-rail and steel-wheel transportation systems including freight, passenger and mass transit systems suffer from the emission of high noise levels and extensive wear of mechanical components such as wheels, rails and other rail components such as ties. The origin of such noise emission, and the wear of mechanical components may be directly attributed to the frictional forces and behaviour that are generated between the wheel and the rail during operation of the system.
In a dynamic system wherein a wheel rolls on a rail, there is a constantly moving zone of contact. For purposes of discussion and analysis, it is convenient to treat the zone of contact as stationary while the rail and wheel move through the zone of contact. When the wheel moves through the zone of contact in exactly the same direction as the rail, the wheel is in an optimum state of rolling contact over the rail. In such a case, no appreciable friction exists between the wheel and the rail. However, because the wheel and the rail are profiled, often misaligned and subject to motions other than strict rolling, the respective velocities at which the wheel and the rail move through the zone of contact are not always the same. This is often observed when fixed-axle railcars negotiate curves wherein true rolling contact can only be maintained on both rails if the inner and the outer wheels rotate at different peripheral speeds. This is not possible on most fixed-axle railcars. Thus, under such conditions, the wheels undergo a combined rolling and sliding movement relative to the rails. Sliding movement may also arise when traction is lost on inclines thereby causing the driving wheels to slip.
The magnitude of the sliding movement is roughly dependent on the difference, expressed as a percentage, between the rail and wheel velocities at the point of contact. This percentage difference is termed creepage.
At creepage levels larger than about 1%, appreciable frictional forces are generated due to sliding, and these frictional forces result in noise and wear of components (H. Harrison, T. McCanney and J. Cotter (2000), Recent Developments in COF Measurements at the Rail/Wheel Interface, Proceedings The 5th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems CM 2000 (SEIKEN Symposium No. 27), pp. 30–34, which is incorporated herein by reference). The noise emission is a result of a negative friction characteristic that is present between the wheel and the rail system. A negative friction characteristic is one wherein friction between the wheel and rail generally decreases as the creepage of the system increases in the region where the creep curve is saturated. Theoretically, noise and wear levels on wheel-rail systems may be reduced or eliminated by making the mechanical system very rigid, reducing the frictional forces between moving components to very low levels or by changing the friction characteristic from a negative to a positive one, that is by increasing friction between the rail and wheel in the region where the creep curve is saturated. Unfortunately, it is often impossible to impart greater rigidity to a mechanical system, such as in the case of a wheel and rail systems used by most trains. Alternatively, reducing the frictional forces between the wheel and the rail may greatly hamper adhesion and braking and is not always suitable for rail applications. In many situations, imparting a positive frictional characteristic between the wheel and rail is effective in reducing noise levels and wear of components.
It is also known that, wear of train wheels and rails may be accentuated by persistent to and fro movement resulting from the presence of clearances necessary to enable a train to move over a track. These effects may produce undulatory wave patterns on rail surfaces and termed corrugations. Corrugations increase noise levels beyond those for smooth rail-wheel interfaces and ultimately the problem can only be cured by grinding or machining the rail and wheel surfaces. This is both time consuming and expensive.
There are a number of lubricants known in the art and some of these are designed to reduce rail and wheel wear on rail roads and rapid transit systems. For example, U.S. Pat. No. 4,915,856 discloses a solid anti-wear, anti-friction lubricant. The product is a combination of anti-ware and anti-friction agents suspended in a solid polymeric carrier for application to the top of a rail. Friction of the carrier against the wheel activates the anti-wear and anti-friction agents. However, the product does not display a positive friction characteristic. Also, the product is a solid composition with poor retentivity.
There are several drawbacks associated with the use of compositions of the prior art, including solid stick compositions. First, outfitting railcars with friction modifier stick compositions and applying to large stretches of rail is wasteful if a noise problem exists at only a few specific locations on a track. Second, some railroads have a maintenance cycle that may last as long as 120 days. There is currently no stick technology that will allow solid lubricant or friction modifiers to last this period of time. Third, freight practice in North America is for freight cars to become separated all over the continent, therefore friction modifier sticks are required on many if not all rail cars which would be expensive and impractical. Similarly, top of rail friction management using solid sticks requires a closed system to achieve adequate buildup of the friction modifier product on the rail. A closed system is one where there is essentially a captive fleet without external trains entering or leaving the system. While city transit systems are typically closed, freight systems are typically open with widespread interchange of cars. In such a system, solid stick technology may be less practical.
U.S. Pat. Nos. 5,308,516, 5,173,204 and WO 90/15123 relate to solid friction modifier compositions having high and positive friction characteristics. These compositions display increased friction as a function of creepage, and comprise resins to impart the solid consistency of these formulations. The resins employed included amine and polyamide epoxy resins, polyurethane, polyester, polyethylene or polypropylene resins. However, these require continuous application in a closed loop system for optimal performance.
European Patent application 0 372 559 relates to solid coating compositions for lubrication which are capable of providing an optimum friction coefficient to places where it is applied, and at the same time are capable of lowering abrasion loss. However, the compositions do not have positive friction characteristics. Furthermore, there is no indication that these compositions are optimized for durability or retentivity on the surfaces to which they are applied.
Many lubricant compositions of the prior art are either formulated into solid sticks or are viscous liquids (pastes) and thus may not be applied to sliding and rolling-sliding systems as an atomized spray. The application of a liquid friction control composition in an atomized spray, in many instances reduces the amount of the composition to be applied to a rail system and provides for a more even distribution of the friction modifier composition at the required site. Furthermore, atomized sprays dry rapidly which may lead to minimizing the potential for undesired locomotive wheel slip.
Applying liquid-based compositions to the top of the rail has distinct advantages over using a solid stick delivery system applied to the wheels. Using a liquid system allows for site-specific application via a hirail, wayside or onboard system. Such specific application is not possible with the solid delivery system that continually applies product to the wheels. Furthermore the low transference rate of the solid stick application method will not yield any benefits until the track is fully conditioned. This is an unlikely situation for a Class 1 rail line due to the extensive amount of track that must be covered and the presence of rail cars not possessing the solid stick lubricant. Liquid systems avoid this problem as the product is applied to the top of the rail, allowing all axles of the train to come in contact with, and benefit immediately from the product. However, this is not always true as the ability of the applied film to remain adhered to the rail and provide friction control is limited. Under certain conditions liquid products have worn off before a single train pass.
WO 98/13445 describes several water-based compositions exhibiting a range of frictional compositions including positive frictional characteristics between two steel bodies in rolling-sliding contact. While exhibiting several desirous properties relating to frictional control, these composition exhibit poor retentivity, and do not remain associated with the rail for long periods of time, requiring repeated application for optimized performance. These compositions are useful for specific applications, however, for optimized performance repeated re-application is required, and there is an associated increase in cost. Furthermore, due to several of the characteristics of these liquid compositions, these compositions have been found to be unsuitable for atomized spray applications.
While a number of friction modifiers in the prior art exhibit positive friction characteristics, a limitation of these friction modifiers is their inability to be retained on the steel surface and remain effective over prolonged periods. In fact, friction modifiers must be repeatedly applied to the rail head or flange interface to ensure proper friction control and such repeated application can result in substantial costs. Thus, there is a need for friction modifier compositions which exhibit improved retentivity, durability and function over prolonged periods. Such compositions may be effectively used in open in either closed or open rail systems. These compositions may include solid, paste or liquid formulations.
It is an object of the present invention to overcome drawbacks of the prior art.
The above object is met by a combination of the features of the main claims. The sub claims disclose further advantageous embodiments of the invention.